1. Field of the Invention
This invention relates to control systems and methods for controlling inductive loads. More particularly, the invention relates to a low-noise and highly linear output stage for pulse-width-modulation (PWM) amplifiers.
2. Description of Related Art
Switching servo amplifiers are commonly used to supply drive current to inductive loads, such as linear, voice-coil, and DC motors. Such amplifiers often employ a type of output stage commonly known as H-bridge amplifiers, or simply "H-bridges. "
FIG. 1 depicts a conventional H-bridge 100. H-bridge 100 includes four transistors M1-M4 configured to drive an inductive load 110. Each transistor M1-M4 has a corresponding diode D1-D4 connected, in a reverse-current direction, from source to drain. Diodes D1-D2 are typically fabricated integrally with respective transistors M1-M4. The gate voltages of transistors M1-M4 are controlled by driver-amplifier circuits (not shown) of a conventional switching servo amplifier to alternate the direction of current flow through load 110. Turning transistors M1 and M4 on and M2 and M3 off causes current to flow in one direction; turning transistors M1 and M4 off and M2 and M3 on causes current to flow in the other direction. Alternating between transistor pairs causes each terminal of load 110 to alternate between ground potential and the voltage level on power terminal +HV. The transistors are not switched simultaneously: some small delay ensures that transistors M1 and M2 (and similarly M3 and M4) do not conduct at the same time.
Conventional H-bridges and their associated circuitry are well known. An explanation of their operation is therefore omitted for brevity. For further information explaining the operation of several conventional H-bridge configurations, see the following U.S. Patents, the contents of which are incorporated herein by this reference:
4,581,565 to Van Pelt, et al., issued Apr. 8, 1986;
4,851,753 to Hamilton, issued Jul. 25, 1989;
4,873,618 to Fredrick et al., issued Oct. 10, 1989;
5,552,683 to Dargent, issued Sep. 3, 1996; and
5,596,446 to Plesko, issued Jan. 21, 1997.
Conventional H-bridge circuits are too noisy and produce excessive distortion for some precision applications. One facet of this noise is a "ringing" of the voltage levels on load terminals for a time after one pair of transistors is switched on and the alternate pair is switched off.
FIG. 2 illustrates the ringing of an output voltage level on terminal 120 of conventional H-bridge 100. The first segment of the waveform (e.g., T0 to T1) represents the time during which transistors M3 and M2 are on, causing the voltage on terminal 120 to approach ground potential (e.g., zero volts).
Transistors M3 and M2 are switched off and transistors M1 and M4 switched on at time T1. Upon switching, the voltage on terminal 120 exceeds the supply voltage +HV for an instant due to the inductive "kick" of load 110 and associated leads. This noise spike is depicted as a spike 200. Then, after some ringing 210, the voltage on terminal 120 finally settles to the supply voltage +HV. H-bridge 100 also exhibits a noise spike 220 and ringing 230 at time T2 when transistors M3 and M2 are switched on and transistors M1 and M4 are switched off.
Diodes D1-D4 do much to limit the energy of spikes 200 and 220 and associated ringing 210 and 230. Nevertheless, there remains a level of noise that is unacceptable for certain high-performance applications. There is therefore a need for a low-noise, highly linear output stage for pulse-width-modulation amplifiers.